Mass transport kinetics

Mauger et al. [35] used the rotating filter assembly to assess the mass transport kinetics of particle populations of a steroid and demonstrated the applicability of a proposed diffusion model used to interpret the data. 

If the electrochemical kinetics of the process are facile then the overall process will be dominated entirely by mass transport. Kinetic parameters such as the exchange current cannot, therefore, be obtained from such a system by analysis of the cyclic voltammetric response. Systems which satisfy this condition are normally referred to as reversible . This is slightly unfortunate... 

The breakthrough curves measured for the monolithic columns with different proteins are very sharp and confirm again the fast mass transport kinetics of the monoliths [133, 134]. The frontal analysis used for the determination of the breakthrough profile can also be used for calculation of the dynamic capacity of the column. For example, the capacity for the 60x16mm i.d. monolith at 1% breakthrough is 324 mg of ovalbumin and represents the specific capacity of 40.0 mg/g of separation medium or 21.6 mg/ml of column volume. 

Keywords Aqueous multiphase catalysis Regioselective hydrogenation Hydrodynamics Mass transport Kinetic modelling... 

Similarly, phenothiazine may be oxidized to the cation radical species which then dimerizes forming the 3,10 -diphenothiazinyl species (Tsujino, 1969). The product of the electron-transfer step may react, via a second-order process, with a species in solution to form a new product. An example of this type of mechanism involves the reduction of anthraquinone and its derivatives in the presence of oxygen (Jeziorek etal., 1997). To understand quantitatively an EC and EC2 process, the concentration and scan-rate dependence of the associated cyclic voltammograms is matched with theory deriving from the mass transport/kinetic equations for each species. 

It is impossible to understand catalyst deactivation without knowing something about the interactions of reaction and mass transport kinetics. 

The magnitude of the response current of the amperometric glucose biosensor shown in Figure 7.1 depends on four main factors. First, mass-transport kinetics determine the rates at which substrates can be supplied to, and products removed from, the reaction layer in which enzyme is trapped. Second, enzyme kinetics... 

For optical transducers, the measured signals are directly proportional to [P], so that, once again, reaction layer thickness and mass-transport kinetics determine the sensitivity of the biosensor, and signals are directly proportional to analyte concentration. For potentiometric transducers, signals are proportional to log[P], and therefore to log[S]. ... 

The epoxide groups of a polyglycidyl methacrylate-co-ethylene dimethacrylate monolith readily react with diethylamine to create a weak anion exchange column. This monolithic column is characterized by fast mass transport kinetics leading to excellent resolution. The columns seem to be stable, and the specific capacity is 40-mg/g medium for ovalbumin [60,62]. More recently, graft polymerization within... 

The key point is that, in general, the phase behavior of a given reaction system wiU not be known prior to the development of that process. Moreover, in those cases where data are available in the literature, they often refer to mixtures far more dilute than would be used in a commercial process. In such a process, energy and plant costs will clearly dictate that the reaction mixture should contain the minimum amount of SCF (see Beckman s constraint 2 e). This contrasts with SCF extraction, where the concentration of the extract dissolved in the SCF is determined, at least in part, by the mass-transport kinetics on the matrix material. All of these factors mean that the phase behavior of the reaction mixture wiU usuaUy have to be determined by experimental methods. 

In this form the model is rarely used, because the band broadening is not only influenced by axial dispersion, but also from mass-transport kinetics. The mass... 

Role of mass transport kinetics 2.3.2.1 -Limiting current... 

When mass transport kinetics interferes, the steady-state current-potential curves show a limiting current for the redox process, which brings into play this mass transport. The limiting current is shown on a current-potential curve as a horizontal plateau. It is often also called a limiting diffusion current. It is dependent on the mass transport parameters (migration, diffusion and convection) and is, with a few exceptionsproportional to the concentration of the consumed species, because their mass transport limits the reaction rateand therefore the current flow... 

The steady-state current-potential curves reflect the influence of the mass transport kinetics of the eiectroactive species by showing current piateaus, whatever the redox kinetics is. The concentration profiies of the eiectroactive species account for this particuiar situation, as iiiustrated in figure 4.25 for the Fe VFe coupie. 

When the overvoltage is sufficiently low, then it can be divided into the sum of two terms which are frequently called the activation overpotential, ria, on the one hand, related to charge transfer kinetics and on the other hand the concentration overpotential, 7d, related to mass transport kinetics ... 

These models combine Butler-Volmer reaction kinetics with mass transport kinetics in a simple approach [16, 45]. The mass transport between the gas phase in... 

An interesting feature of the system is the very low polarization compared to other conversion electrodes (Fig. 3.8, inset). This has been attributed to the mass transport kinetics where the mobility of the ionic species plays an important role. Hydride ions are supposed to move faster than 0 and F and it is believed thatthere are lower kinetic barriers in the case of hydride systems. But as the role of anion diffusion in the rate-determining step has not been elucidated in detail, the statement remains to be verified. 

Electrochemical ac or direct current (dc) pulse techniques applied on the simple electrochemical system Li/Li+, PC/TiS2 (where PC stand for propylene carbonate) initially corroborated the Randles model, that describes the lithium insertion as a dissolution reaction of the pair (Li+, e ) in the material host. By taking into account the mass transport kinetics of the lithium in the oxide, this famous model has permitted to suggest that the observed electrochemical behavior was correlated to the structure of the host material. However, this model is not complex enough to describe the phenomena that occur at numerous other electrode/electrol3Ae interfaces. In particular, the responses obtained by electrochemical... 

Looking at the template-assisted preparation techniques, the physicochemical properties of the in-situ formed carbon could be of lower quality compared to commercial carbon supports. For fuel cell application, an optimization with respect to (1) the electrical conductivity, (2) the agglomerate or particle size, and (3) the transport properties (hydrophUicity) is necessary. However, optimal physicochemical properties of the carbon might not be obtained at the maximum of volumetric current density (determined by So and TOF). Therefore, approaches to improve catalysts with an in-situ formed carbon should be also done under utUization of FC measurements in order to evaluate the mass transport kinetics. Nevertheless, optimization remains challenging as a good balance between carbon properties and... 

PEM fuel cell characteristics are generally described with polarization curves. The thermodynamic equilibrium potential of the hydrogen/oxygen reaction is reduced by various overvoltage terms that depend on mass transport, kinetic, and ohmic phenomena within cell. In other words, the output voltage of a single cell is attributable to different current, temperature, and pressure dependant factors [1]. 

Baths have been used to apply ultrasound during the osmotic dehydration of apples in sugar solutions (Simal et al, 1998, 2006), cheese (Sanchez et al., 1999) or meat brining (Simal et al., 2006), and also to study its effects on mass transport kinetics. Ultrasound has also been applied in liquid-solid systems as a pretreatment prior to osmotic dehydration or hot-air drying of products such as banana (Fernandes and Rodrigues, 2007), pineapple (Fernandes et al., 2008) or malay apple (Oliveira et al., 2011). 

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